The legal firm Wragge & Co have put together a useful guide:
‘Purdah’ is a political convention, which formally applies to government ministers and civil servants in central government during the period immediately before a general election.
During a period of purdah, ministers and civil servants will refrain from taking decisions or making policy announcements which are significant and may be politically contentious.
But what does that mean in practice? Go and read the guide to find out. Short version: the laws and rules aren’t that clear. If in doubt ask a lawyer. Surprising that from a law firm 
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It's PURDAH time: Find out what this means for you and your customers here http://ow.ly/1v2vX
Purdah also applies to the Armed, Reserve and Cadet Forces.